Selection and Iteration: Mixed Unit Review (Computer Science A Unit 2)

Read the big ideas and the method table, then try the seven questions without notes. Each question combines two or more Unit 2 topics.

These are original Marlbridge practice questions, not past exam questions. The rubrics are a suggested Marlbridge rubric, not official scoring. Assume every code segment compiles unless a question says otherwise. No calculator is allowed.

Big ideas of the unit

• Algorithms are built from three blocks. Sequencing, selection and repetition, and the order and nesting of the blocks decide the outcome (2.1).
• Conditions are Boolean expressions. Relational operators compare values (2.2); !, && and || combine them, with short-circuit evaluation (2.5); truth tables and De Morgan’s laws prove equivalence, and == compares references for objects (2.6).
• Selection chooses a path. if and if-else (2.3), nested ifs and else-if chains, where the first true condition wins (2.4).
• Iteration repeats. A while loop tests before every pass, so it may run zero times or for ever (2.7); a for loop gathers the initialization, test and update, and can be rewritten as a while loop (2.8).
• Standard algorithms combine the two. Divisibility, digits, counting, minimum or maximum, sum or average (2.9) and string algorithms with substring windows (2.10).
• Nested loops multiply or sum. The inner loop completes for each outer pass (2.11).
• Counting executions compares algorithms. Count the statements, and see how the count changes as the input grows (2.12).

Key relationships and methods

Question 1 (multiple choice · 2.5 and 2.6)

Which code segment is equivalent to the following for all integer values of a and b?

if (!(a > 3 || b < 2)) {
    x = 1;
} else {
    x = 2;
}

• (A) if (a < 3 && b > 2) { x = 1; } else { x = 2; }
• (B) if (a <= 3 && b >= 2) { x = 1; } else { x = 2; }
• (C) if (a <= 3 || b >= 2) { x = 1; } else { x = 2; }
• (D) if (a > 3 && b < 2) { x = 1; } else { x = 2; }

Question 2 (multiple choice · 2.7 and 2.9)

What is printed?

int n = 3874;
int result = 0;
while (n > 0) {
    int d = n % 10;
    if (d % 2 == 0) {
        result = result * 10 + d;
    }
    n = n / 10;
}
System.out.println(result);

• (A) 84
• (B) 48
• (C) 12
• (D) 3874

Question 3 (multiple choice · 2.11 and 2.12)

What are the final value of c and the number of times the condition (i + j) % 2 == 0 is evaluated?

int c = 0;
for (int i = 1; i <= 4; i++) {
    for (int j = 1; j <= i; j++) {
        if ((i + j) % 2 == 0) {
            c++;
        }
    }
}

• (A) c is 6; the condition is evaluated 10 times.
• (B) c is 6; the condition is evaluated 16 times.
• (C) c is 10; the condition is evaluated 6 times.
• (D) c is 8; the condition is evaluated 10 times.

Question 4 (constructed response · 2.3, 2.7 and 2.9)

This question is in the style of a methods-and-control-structures task. A digit pair is two adjacent digits of a positive integer that are equal. For example, 1223 has one digit pair (the two 2s), and 55055 has two (the first two 5s, and the last two).

Write the method equalNeighbours that returns the number of digit pairs in the positive integer n. The method must use a loop and must not convert n to a String. (7 marks)

public static int equalNeighbours(int n)

public static int equalNeighbours(int n) {
    int count = 0;
    int previous = n % 10;
    n = n / 10;
    while (n > 0) {
        int current = n % 10;
        if (current == previous) {
            count++;
        }
        previous = current;
        n = n / 10;
    }
    return count;
}

Question 5 (constructed response · 2.10)

A String names contains one or more words separated by single spaces, with no space at the beginning or end. Write the method initials that returns a String made of the first letter of each word, in order. Use indexOf and substring. (6 marks)

public static String initials(String names)

public static String initials(String names) {
    String result = "";
    String rest = names;
    while (rest.indexOf(" ") >= 0) {
        result = result + rest.substring(0, 1);
        rest = rest.substring(rest.indexOf(" ") + 1);
    }
    result = result + rest.substring(0, 1);
    return result;
}

Question 6 (constructed response · 2.4, 2.8, 2.11 and 2.12)

Consider the following code.

for (int r = 0; r < 4; r++) {
    for (int c = 0; c < 6; c++) {
        if (r == 0 || r == 3 || c == 0 || c == 5) {
            System.out.print("#");
        } else {
            System.out.print(".");
        }
    }
    System.out.println();
}

(a) What is printed? (2 marks)

(b) How many times is the condition in the if evaluated? How many times is System.out.print("#") executed? (2 marks)

(c) Write the condition that is true exactly when the else branch runs, using De Morgan’s law, with no ! in front of a bracket. (2 marks)

######
#....#
#....#
######

Question 7 (constructed response · 2.6, 2.11 and 2.12)

Two methods count the pairs of whole numbers a and b, each between 1 and n inclusive, with a + b == s.

public static int pairsNested(int n, int s) {
    int count = 0;
    for (int a = 1; a <= n; a++) {
        for (int b = 1; b <= n; b++) {
            if (a + b == s) {
                count++;
            }
        }
    }
    return count;
}

public static int pairsSingle(int n, int s) {
    int count = 0;
    for (int a = 1; a <= n; a++) {
        int b = s - a;
        if (b >= 1 && b <= n) {
            count++;
        }
    }
    return count;
}

(a) Give the value returned by each method for (n, s) equal to (10, 11), (10, 4) and (10, 25). (3 marks)

(b) For n = 10, how many times is the if condition evaluated in each method? (2 marks)

(c) When n is doubled to 20, by what factor does each method’s number of condition evaluations change? (2 marks)

(d) Write the condition that is true exactly when pairsSingle does not count the value of a, with no ! in front of a bracket. (1 mark)

How did you do?

If a question went wrong, find its topics in the title and go back to that study guide. Use the topic checklists to make sure you can do each skill without notes: 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 2.10, 2.11 and 2.12. Then move on to Unit 3 (Class Creation), where you will write your own classes and methods using the control structures you now know.